Ten tuning forks are set out in order of rising frequency…

(a) Each step up adds $6\ \text{Hz}$, and there are 9 steps from the first fork to the last. So $f_{10} = f_1 + 9 \times 6\ \text{Hz} = f_1 + 54\ \text{Hz}$.

We are told $f_{10} = 1.5f_1$. So $1.5f_1 = f_1 + 54\ \text{Hz}$. We want $f_1$, so $0.5f_1 = 54\ \text{Hz}$ and $f_1 = 108\ \text{Hz}$.

(b) $f_{10} = 1.5 \times 108\ \text{Hz} = 162\ \text{Hz}$.

(c) The fifth fork is 4 steps up: $108\ \text{Hz} + 4 \times 6\ \text{Hz} = 132\ \text{Hz}$.